1. Field of the Invention
The present invention relates to a molding material for use in the preparation of thermoplastic composites containing continuous reinforcing fibers in a thermoplastic resin matrix. In particular, the present invention relates to a molding material formed with commingled yarn containing continuous thermoplastic fibers as well as continuous reinforcing fibers. Furthermore, the present invention also relates to a pelletized molding material for use in the preparation of thermoplastic composites. The reinforcing fibers in the pelletized molding material are aligned in a longitudinal direction within a thermoplastic resin matrix of the molding material. Moreover, the present invention concerns a process for the preparation of thermoplastic composites using the aforementioned molding material.
2. Description of the Prior Art
Existing plastic composites containing reinforcing fibers include the so-called "advanced composites" employed for the manufacture of products used in the aviation and space industries as well as the general purpose fiber reinforced plastics (FRP) used in shipbuilding, for bathtub units and in automobiles, etc. In both of these composites, thermosetting resins are ordinarily employed as matrix resins, usually epoxy resins in the former type and unsaturated polyester resins in the latter. If thermosetting resins are used as the matrix, then hardening treatment is necessary in order to harden the matrix resin, moreover, the composites so obtained lack toughness. On the other hand, it is known that improvement with respect to the aforementioned shortcomings can be achieved by using thermoplastic resins for the matrix resins. Furthermore, composites with excellent impact resistance and fatigue resistance can be obtained by using, for example, glass fibers of a length of 3 mm or more as the reinforcing fibers. However, the melting temperature of thermoplastic resins is high, and therefore high temperatures and high pressures are required in order to impregnate long reinforcing fibers with the molten resin. Since the impregnation of molten resin into a long-fiber mat with randomly oriented fibers is relatively easy, composites are ordinarily obtained by impregnating such mats with a thermoplastic resin and then thermoforming these resin-impregnated mats. However, when this method is employed, a high content of reinforcing fibers cannot be achieved, and therefore the mechanical properties of the resulting composites are inadequate.
With the purpose of solving this problem, Japanese Laid-Open Patent Publication Nos. 60-209034 and 61-130345 disclose processes for the manufacture of molding materials for composites by commingling continuous reinforcing fibers with continuous thermoplastic fibers. These molding materials are molded into composites by techniques such as hot pressing, pultrusion and filament winding, and permit the formation of composites with irregular surfaces. These processes also make possible a marked increase in the reinforcing fiber content of manufactured thermoplastic resin composites as compared with processes based upon impregnation of fiber mats with molten resins. Nevertheless, even when these types of molding materials are employed, adequate quantities of thermoplastic resin cannot be impregnated into the continuous reinforcing fibers when thermoforming is carried out, and consequently the composites so obtained lack toughness. Therefore, a molding material such that the continuous reinforcing fibers are impregnated with a sufficient quantity of thermoplastic resin during the thermoforming process and which thereby provides thermoplastic composites with excellent mechanical properties is desirable.